Pulp is produced from raw wood material and is the basic ingredient in the production of paper. The objective of pulping is to separate and recover cellulose fibers from lignin and other wood constituents with maximum yield and minimum fiber degradation. Kraft pulping is the most common process, and accounts for approximately two-thirds of worldwide pulp production.
The kraft pulping process involves treating or ‘cooking’ wood chips with a mixture of sodium hydroxide (NaOH) and sodium sulfide (Na2S), known as white liquor, to break the bonds that link lignin to cellulose and produce the cellulose fibers used to make paper. Non-fibrous material and spent cooking chemicals are then sent to a recovery process, where the pulping chemicals and energy are recovered via multiple evaporation steps for concentration of pulping waste liquid (black liquor) which is then burned. Large scale evaporators are used in the evaporation steps, and produce condensates which contain substances, some of which have been reported to be toxic when released to the environment (Belknap, A. M., K. R. Solomon, D. L. MacLatchy, M. G. Dubé and L. M. Hewitt. 2006. Environ. Toxicol. Chem. 25(9):2322-2333; Hewitt, L. M., S. A. Smyth, M. G. Dubé, C. I. Gilman and D. L. MacLatchy. 2002. Environ. Toxicol. Chem. 21:1359-1367; and Hewitt, L. M., T. G. Kovacs, M. G. Dubé, D. L. MacLatchy, P. H. Martel, M. E. McMaster, M. G. Paice, J. L. Parrott, M. R. van den Heuvel, and G. J. Van Der Kraak. 2008. Environ. Toxicol. Chem. 27(3):682-697).
Tall oil (also called liquid rosin or tallol) can be collected from the black liquor as a byproduct of the process, and can generate significant revenues when sold for use in products such as adhesives, rubber and emulsifiers. Otherwise, technologies have been developed to remove or reduce remaining pollutants before they are discharged in the mill effluent, for instance by biological treatment to metabolize and consume residual organic material. Apart from tall oil, however, there has been very little attention given to the recovery of natural products of commercial value.
The present invention is therefore directed to recovery methods to extract useful chemicals from pulp and paper waste waters.